Firetubes are tubes used in some steam boilers to convey heated gases from one tube sheet to an opposite tube sheet of a boiler. Heated gases traverse the firetube, conducting heat through the firetube's wall and transferring heat energy to the water that surrounds the firetube. Gases exit the opposite tube sheet at a significantly lower temperature.
Steam boilers capable of producing superheated steam comprise superheater elements having steam flowing within the element tube, and with hot gases within firetubes flowing on the outside of the superheater elements.
A superheater element consists of a superheater tube that conducts the flow of steam into and out of a firetube in order to impart heat energy from the high temperature gases in the firetubes to the saturated steam inside the superheater elements, causing the steam to exit the superheater element with more useful energy per unit volume of steam than if the steam were not superheated.
Currently, most commercial steam boilers 55 are either of the scotch wet-back horizontal firetube type, illustrated by example in FIG. 1A, or the scotch dry-back horizontal firetube type, illustrated by example in FIG. 1B. In these steam boilers, a burner source 3 burns fuel in a relatively large furnace tube and heats the gas therein. The high temperature gases exit the furnace tube, execute a 180 degree turn, and flow through relatively small diameter firetubes stretched between two tube sheets. Tube sheets are plates that secure the pressure boundaries of the firetubes and hold the firetubes in place. The scotch wet-back horizontal firetube boiler illustrated in FIG. 1A has three tube sheets, whereas the scotch dry-back horizontal firetube boiler illustrated in FIG. 1B has two tube sheets.
The current boiler art uses one furnace tube of appropriate diameter to promote the most efficient combustion for the design steaming capacity and as many small diameter tubes as possible to create large surface area to accommodate efficient convective heat transfer rates across the tube walls from the heated furnace gases. In the case of horizontal firetube boilers, illustrated in FIGS. 1A and 1B, the furnace tube and the many small diameter gas firetubes are surrounded by water in a steel cylinder boiler designed to withhold the design boiler pressure. As the high temperature gases flow through the one large diameter furnace tube and multiple smaller diameter firetubes, the high temperature gases give up heat to boil water 11 inside the pressure boundaries of the boiler. The saturated steam of the boiling water 11 collects in the steam space at the top portion of the boiler and exits through a valve at the top wall of the boiler.
Some boilers are designed to circulate the heated furnace gases several times back and forth through different banks of tubes, called “passes,” in order to extract as much heat as possible before exhausting the gases out the smokestack 50 to the atmosphere. Boilers of the locomotive type combust the fuel in a firebox and exhaust the gases after only one pass through the firetubes. The scotch wet-back horizontal firetube steam boiler shown in FIG. 1A has three passes and the scotch dry-back horizontal firetube steam boiler shown in FIG. 1B has two passes.
The steam generating capacity of a given boiler is dictated by the size of the space the boiler can occupy. The boilers are typically cylindrical, being the strongest practical shape to contain pressurized fluids. Greater steam generating capacity is achieved by making the boiler shells larger in diameter and increasing the distance between the tube sheets.
Efficiency of the boiler is increased by diverting the gases through several passes to increase the tube surface area the gases are exposed to before exhausting the heated gases through smokestack 50.
The laws of physics regarding heat transfer and gas flow dictate the cross-sectional area for a given firetube to achieve the most efficient combustion and heat transfer. Firetubes with smaller diameters have less volume for the high temperature gases to flow through but have greater surface area to volume ratios which means more surface area to absorb heat. Optimal firetube efficiency is achieved by balancing the amount of hot gases flowing in a given period of time verses the overall surface area for heat transfer.
Superheated steam at a given pressure has a higher temperature than the temperature at which water boils at that same pressure. For example, at 14.7 pounds per square inch (1 bar) (sea level), superheated steam would have a temperature higher than 212° F. (100° C.), which is the temperature of regular saturated steam from boiling water at that pressure; or at 150 pounds per square inch (10 bar), which is approximately ten times sea level atmospheric pressure, superheated steam will have a temperature higher than 366° F. (186° C.), which is the temperature of regular saturated steam from boiling water at that pressure. To superheat steam, it must be collected from the boiler and subjected to additional heat input from either an external heat source or the furnace gases.
The advantage of superheated steam is the ability to transfer more thermal energy from the boiler source to the destination at a given pressure with less boiled water. This allows more energy to be transmitted with the same amount of steam without increasing pressure or the infrastructure of the piping system.
Superheating steam in firetube boilers is well known in the art. Typical prior art embodiments comprise adding significantly larger firetubes in the boiler, with a small diameter superheater tube filled with steam passing down within a single firetube from one end and a small radius u-bend in the superheater tube to send the steam back out the same firetube in the opposite direction. The superheater tubes reverse direction inside the large diameter tubes at least once, and in some embodiments twice. FIG. 2A depicts an example of a one-directional flow firetube superheater with multiple u-turns or passes. A portion on the left of the one directional flow firetube superheater tube is cut out to illustrate the inside structure of one of the u-turn bends in the superheater tube.
Among the disadvantages of these prior art one-directional flow superheater tubes with one or more u-turn bends are:                They require large diameter firetubes, lowering the total number of firetubes that can be utilized in a given diameter boiler for a specific sized boiler shell. The total heating surface for water to cause steam generation is thereby reduced, reducing the boiler steam generating capacity.        As the steam flows through the superheater element making multiple passes through the firetube, with each pass being from the low temperature end to the high temperature end of the firetube and then back to the low temperature end, the temperature of the furnace gases drop exponentially. The steam in the outbound superheater tube, being heated to a high degree in the high temperature end of the firetube, has a higher temperature than the gases in the low temperature end of the firetube. At a certain point along the firetube, the steam in the outbound superheater tube will have a higher temperature than the continually cooling adjacent furnace gases. At this point, the superheating process becomes counterproductive as the superheater tube is now giving up heat to the furnace gases that are exiting the firetube, possibly to the exhaust of the boiler. This thermodynamic effect is illustrated in the prior art firetube boiler schematic of FIG. 2B, and explained in more detail in below.        
An object of this subject invention is to provide a superheater element that overcomes the disadvantages of the currently available superheater elements.
A further object of the current invention is to superheat steam without requiring an additional external heat source to heat the saturated steam into superheated steam.
A further object of the current invention is to superheat steam using the furnace gases used to heat the boiler water into saturated steam.
A further object of the current invention is to provide a superheater element that produces superheated steam more efficiently than currently available superheater elements.
A further object of the subject invention is to provide a superheater element wherein the superheated steam circulating within the firetube does not lose any heat energy to the firetube gases.
A further object of the subject invention is to provide a superheater element that uses fuel more efficiently than currently available superheater boilers.
A further object of the subject invention is to provide a superheated boiler that can produce more superheated steam with less fuel, produce superheated steam at a higher temperature with the same amount of fuel, or produce superheated steam having the potential to do more work with the same amount of fuel.
A further object of the subject invention is to provide an improved superheater element that can be easily and inexpensively retrofitted into conventional firetube boilers.